W3.org:

W3.org has 287 URLs listed in 163 Dmoz categories.

XML-Data
Describes an XML vocabulary for schemas, that is, for defining and documenting object classes. [W3C Note 05 Jan 1998]
http://www.w3.org/TR/1998/NOTE-XML-data/
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/Historical_Schema_Languages

SOX - Schema for Object-Oriented XML 2.0
A schema language for defining the syntactic structure and partial semantics of XML document types; an alternative to XML DTDs. [W3C Note 30 July 1999]
http://www.w3.org/TR/NOTE-SOX/
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/Historical_Schema_Languages

W3C Official Public Site for XML Schema
Contains status information, links to specifications and tools that implement XML Schema
http://www.w3.org/XML/Schema
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema

W3C XML Schema Test Collection
Collection of test schemas used to ensure consistency among XML Schema implementations.
http://www.w3.org/2001/05/xmlschema-test-collection.html
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema

XML Schema: Formal Description
Formalization of a formal, declarative system for describing and naming XML Schema information, specifying XML instance type information, and validating instances against schemas. (W3C Working Draft, 25 September 2001)
http://www.w3.org/TR/xmlschema-formal/
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Standards_Documents

XML Schema Part 2: Datatypes
Part 2 of the specification of the XML Schema language. It defines facilities for defining datatypes to be used in specifications. (W3C Recommendation 2 May 2001)
http://www.w3.org/TR/xmlschema-2/
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Standards_Documents

XML Schema Part 0: Primer
Part of the official XML Schema definition. A non-normative document intended to provide an easily readable description of the XML Schema facilities. (W3C Recommendation 2 May 2001)
http://www.w3.org/TR/xmlschema-0/
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Standards_Documents

XML Schema Part 1: Structures
Specifies the XML Schema definition language, which offers facilities for describing the structure and constraining the contents of XML 1.0 documents. (W3C Recommendation 2 May 2001)
http://www.w3.org/TR/xmlschema-1/
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Standards_Documents

XML Schema Requirements
Describes the requirements on which the W3C XML Schema recommendation is based. (W3C Note 15 February 1999)
http://www.w3.org/TR/NOTE-xml-schema-req
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Standards_Documents

Errata in XML Schema
List of errata in the W3C XML Schema recommendation of 2 May 2001
http://www.w3.org/2001/05/xmlschema-errata
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Standards_Documents